This invention relates to a spin coating apparatus for forming a thin film of a coating liquid, such as a photoresist, on the surface of a semiconductor substrate.
In the manufacture of semiconductor devices, it is necessary to form coatings of various chemicals, such as photoresists, on the surfaces of semiconductor substrates. The most commonly used method is the spin coating method in which a liquid chemical, referred to as a coating liquid, drips from a nozzle onto the center of the top surface of a substrate. The substrate is then rotated about its center, and centrifugal force causes the drops of coating liquid to spread outwards over the surface of the substrate and form a coating.
Some of coating liquids for use in spin coating contain thermosetting materials or thermal crosslinking agents, many of which begin to set at room temperature (20.degree.-25.degree. C.). Spin coating is usually performed at room temperature, and frequently, the coating liquid is stored at room temperature prior to use. However, if the coating liquid is stored at room temperature, its viscosity will gradually increase over time. As the thickness of the coating which is formed by spin coating depends on the viscosity of the coating liquid, the increase in the viscosity of the coating liquid makes it impossible to maintain a constant coating thickness. Furthermore, the coating liquid beings to set at room temperature within the piping which supplies the coating liquid to the nozzle, and the piping and the nozzle can become clogged by the coating liquid.
In order to solve this problem, it is conceivable to cool the entire spin coating apparatus to a temperature below room temperature at which the coating liquid will not begin to set. However, this leads to a large increase in operating costs and is impractical. Another possible method of preventing the coating liquid from setting is to cool only those parts of the spin coating apparatus which come into contact with the coating liquid prior to its application to a substrate (such as a reservoir for storing the coating liquid, the nozzle through which the coating liquid is applied, and piping leading from the reservoir to the nozzle) while the other portions of the spin coating apparatus are maintained at room temperature. However, with this method, the coating liquid which is applied to the substrate by the nozzle is at a lower temperature than the substrate, which is at room temperature. In spin coating, if the temperature of the coating liquid is different from the temperature of the substrate to which it is applied, a coating with a uniform thickness can not be formed.